By their nature, automobile bumpers must be able to withstand considerable impact. Metal or metal reinforced bumpers have heretofore been the only practical alternative for car makers because of the strength and durability associated with the metal components. Unfortunately, metal bumper components are expensive to produce, and add substantial weight to automobiles, decreasing fuel efficiency.
Car makers and automotive parts makers have experimented a great deal with plastic bumpers, seeking a cheaper and lighter weight substitute for the heavier traditional metal bumpers. Though appreciable progress has been made, plastic bumpers still have a strength to weight ratio that is unacceptable to the automotive industry. There are two aspects to the shortcomings of the plastic bumpers. First, the automotive industry has been unable to develop a plastic material that is economically viable with acceptable tensile and impact strength for bumper use. Second, the automotive industry has been unable a develop a plastic frame arrangement capable of withstanding impacts typically encountered in automobile accidents.
Currently, the most popular plastic materials used for automotive parts and bumpers are glass mat thermoplastics. Glass mat thermoplastic (GMT) composites are a family of compression-moldable, fiberglass-reinforced materials with thermoplastic matrices whose mechanical properties are generally higher than those of standard, injection-molded thermoplastic composites. GMT is available in the following glass-mat types: continuous-strand, randomly oriented glass-mat products which provide a good balance of stiffness and strength in all three axes; unidirectional long-glass-fiber mats which add directional stiffness and strength in a single axis; and, long, chopped fiber glass mats which provide improved flow properties and improvements in energy management with minimum decrease in stiffness.
The different glass mats are combined with a thermoplastic resin, usually polypropylene, (although other higher temperature engineering resins are also offered) to form a moldable product. GMT products are supplied in sheet or blank form to processors who shape the materials by compression molding or thermostamping.
Until recently, plastic bumpers have traditionally been manufactured with frame arrangements having a "C" or a "W" cross-section; the "C" or the "W" is used to describe the shape of the plastic cross-members connecting the front and rear plastic bumper walls. These configurations were chosen because of favorable energy absorption characteristics, especially when the bumper was impacted with a vertical component of force. I-section plastic bumper design has evolved over the past 10-years. Reinforced plastic bumpers have provided adequate performance and significant weight reduction but at a cost penalty. In 1992, the publication Plastic News speculated that "A new bumper design concept--the I-beam--offers the potential for making plastic bumper beams even lighter weight and cost competitive with steel bumpers up to volumes in excess of 100,000 per year."
U.S. Pat. No. 5,269,574, to Bhutani et al. discloses a bumper with an I-beam shape constructed of fiber reinforced thermoplastic or thermoset reinforcing resin. The fiber reinforcing the thermoplastic is selected from a group of unoxidizable steel fibers, aluminized glass fibers, cellulosic fibers or glass fibers. Unfortunately, implementation of the design disclosed by Bhutani et al. presents molding problems that are inconsistent with the economic constraints in the automobile industry. For example, the Bhutani et al. I-section bumper structure having ribbed sections fails to form adequately during molding. X-ray analysis shows that only 50 to 80% of the fibers used in the molding of the Bhutani et al. bumper flow into the ribs. The economic considerations require that mounting stays be integral to the structure.